This invention relates to the manufacture of soybean curd, and more praticularly to an improved process for the manufacture of soybean curd such as the type called "tofu" in Japan.
The soybean curd called "tofu" is one of the typical Japanese foods, which has been widely used since old times and is currently very popular in Japan. Also, in the United States, European and other countries, the soybean curd has come to be regarded as an attractive food because it contains high quality protein and is available at a low price.
The soybean curd now available in the markets under the name of "tofu" (excluding such special ones as those baked, frozen and dried, or fried) may be roughly classified into four types, that is, "momen tofu" (which substantially means a cotton strained tofu), "kinugoshi tofu" (which substantially means a tofu having silky body surfaces), "soft tofu", and "packed tofu".
The type called "momen tofu" is generally manufactured by putting prepared crushed soybean milk gel into an open-top, perforated mold vessel having its perforated walls internally covered with a water-permeable cotton cloth (cotton gauze) and then molding the same in the vessel into a mass by applying an adjusted pressure thereupon, whereby surplus water flows out through the cotton cloth as well as the perforated walls, and thus is removed from the vessel. Finally, the obtained mass is cooled and then cut into a desired number of cubic pieces which are final products. The "momen tofu" thus produced is generally lower in water content than any one of the other three types of "tofu".
The type called "kinugoshi tofu" is generally manufactured by pouring prepared soybean milk into an open-top, non-perforated mold vessel and then mixing it in the vessel with a coagulant into a coagulated mass, without dewatering. The obtained mass is cooled and then cut into a desired number of cubic pieces which are final products. Because of the lack of dewatering, the "kinugoshi tofu" is more watery and softer than the "momen tofu" and has very smooth and silky body surfaces while being very easily broken into pieces.
The type called "soft tofu", which is a relatively modern type of "tofu", is generally manufactured by pouring prepared soybean milk into an open-top, perforated mold vessel having its perforated walls internally covered with a water-permeable cotton cloth together with an overlapping, water-impermeable sheet made of synthetic resin, and then mixing it with a coagulant to effect coagulation. When the resultant mixture is coagulated to a certain extent, the water impermeable sheet is removed, so that a certain amount of water contained in the mixture flows out through the water-permeable cotton cloth as well as the perforated walls and thus is removed from the vessel. Then, the coagulation is continued to its full extent under an adjusted pressure, until a desired, fully coagulated mass is obtained. Finally, the obtained mass is cooled and then cut into a desired number of cubic pieces which are final products. The "soft tofu" this produced has intermediate properties between those of the "momen tofu" and those of the "kinugoshi tofu".
The type called "packed tofu", which is also a relatively modern type of "tofu", is generally manufactured by pouring prepared soybean milk (preferably cooled down to about 4.degree.-15.degree. C.) together with an added coagulant into a given number of film-made cubic containers, vacuum-packing the soybean milk in the containers, heating the same into a coagulated gel, and then cooling down the same after full coagulation, whereby vacuum-packed final products are obtained.
As apparent from the above description, the four types of soybean curd are somewhat different from one another in the process of manufacture. However, it is common to all of the four types of soybean curd that prepared soybean milk is coagulated into a gel by means of a coagulant.
As such a coagulant, calcium sulfate is now most extensively used for the current mass production of the aforedescribed four types of soybean curd, while, in some cases, gulco-delta-lactone and calcium chloride are also used for the same purpose.
On the other hand, it is well known in the field to fee manufacture invention that natural bittern (magnesium chloride) was used as the coagulant many years ago. It is also well known that it is theoretically possible to produce commercially valuable soybean curd which is quite excellent in flavor, adequately soft and elastic, very good in taste, mouth feel, body texture, and surface luster, provided that bittern is used as the coagulant under certain proper conditions.
Nevertheless, no one knows the details as to how and under what conditions bittern was added to soybean milk in such old days. In fact, it has proved technically very difficult to use bittern (magnesium chloride) as the coagulant for the manufacture of soybean curd on a mass production scale, because magnesium chloride contained in the bittern reacts so quickly with soybean milk into local or heterogeneous coagulation, as compared to any one of the currently used coagulants such as calcium sulfate, gluco-delta-lactone and calcium chloride, resulting in the produced soybean curd often being too hard, not uniform in quality, poor in water retention, and/or dissatisfactory in surface luster, body texture and mouth feel. In other words, it is considered the use of bittern (magnesium chloride) as the coagulant is always accompanied by high skill and accumulated experience, which is a fatal disadvantage in the manufacture of soybean curd on a mass production scale under an automatic system at a low manufacturing cost. This is why the above-mentioned calcium sulfate, gluco-delta-lactone and calcium chloride have taken the place of bittern (magnesium chloride) as the coagulant in the current manufacture of soybean curd on a mass production scale.
However, it is clear that none of the above-mentioned, currently used coagulants can exceed bittern (magnesium chloride) from the viewpoint producing high quality of products which taste good. In addition thereto, it is presumed that calcium sulfate (55-120 mg of calcium sulfate is generally contained per 100 g of product soybean curd) is not good for the health of human beings. Further, in the case where the above-mentioned currently used coagulants are employed, dispersing of seasonings such as soy sauce into a product body has proved to be relatively poor as compared to the case where magnesium chloride is employed.
Therefore, various attempts have been made to produce good-tasting, high quality soybean curd by using magnesium chloride as the coagulant, as disclosed, for example, in Japanese Patent Publication No. 56-39866, published Sept. 16, 1981 (Patent Application No. 53-48253, filed Apr. 21, 1978, Seiich KUBOTA as Inventor and Applicant). This prior art is concerned with an improved process for manufacturing the two types of soybean curd called "kinugoshi tofu" and "soft tofu" by using magnesium chloride as the coagulant. More particularly, this prior art method comprises the steps of: (1) pouring prepared soybean milk into an agitating bath and producing a certain flow (preferably one way flow) of the soybean milk in the bath; (2) adding magnesium chloride to the flowing soybean milk (6-9 g of MgCl.sub.2 in powder or aqueous solution/1 l of soybean milk) to obtain a flowing state of mixture thereof; (3) transferring the obtained mixture into a mold vessel and then leaving the same for about one hour until a coagulated mass is produced; and (4) pouring a prepared, heated aqueous solution of 40% MgCl.sub.2 over the coagulated mass.
This prior art method, however, is disadvantageous in the following points: the first disadvantage being that the method is not applicable to manufacture of soybean curd of the type called "momen tofu" because the process for the "momen tofu" requires such particular steps of crushing a coagulated gel of the mixture of soybean milk and the added magnesium chloride and then transferring the resulted crushed gel into a perforated mold vessel for molding the same into a mass under pressure. The second disadvantage being that local or heterogeneous coagulation of a mixture of soybean milk and the added magnesium chloride is inevitably invited because the coagulating reaction of the magnesium chloride with the soybean milk is excessively accelerated when the mixture thereof is in the continued flowing state. The third disadvantage being that manufacturing efficiency is poor because the method requires such a step of leaving the resulted mixture for about one hour in the mold vessel. The fourth disadvantage being that it is considered that, if as much as 6-9 g of MgCl.sub.2 is added per 1 l of the flowing soybean milk, the resultant mixture is likely coagulate before it is transferred into the mold vessel. Thus, this prior art method may often be unpractical for the manufacture of the desired good-tasting, high quality soybean curd on a mass production scale.